The cast spent two days filming in the famous children's hospital and struggled to watch ill patients rushed into the wards.

"It's such a lovely place, it's got such a friendly vibe. They chose a day when they knew a lot of people wouldn't be in for operations in the ward we were in but we did have to stop a few times when children were being brought through for various different wards and it was just gut-wrenching. Sometimes, some of the crew would cry and it was just so moving," she said.

Fraser, who has a nine-year-old daughter Lila with her husband, Irish-born actor Karl Geary, said tackling the role of a mother who facing losing her young child was a cathartic experience.

"Morbid as it sounds, it's quite a cathartic experience playing a role like that because you can act out all your own fears and hopefully get rid of them for a while. I always have so much worry as a mother that I just dwelled on my usual worry and exaggerated it for the duration of the job."

She added: "It was always great when I got to come home and cuddle (Lila) and she was jumping about and was not attached to any tube. God, it makes you so grateful."

Luckily, the cast was full of comedians to provide light relief - although it made her and co-star Tucci "crack up a lot" during some of their more serious scenes.

Tucci, 55, plays the triple role of heart surgeon Mr Wiley, as well as her husband Mr Darling and Peter's pirate nemesis, Captain Hook.

"Stanley's really funny, very cool, he's got so much confidence. You would definitely fancy him if he was your doctor! He was really fun to work with, just an absolute hoot," she revealed.

"We got cracked up by all the comedians that are playing the pirates. It was really hard because I was trying to look really upset in a scene with all the pirates and I just couldn't stop laughing," she said.

The part is one in a list of prominent roles for Breaking Bad star Fraser, who has managed to buck the entertainment industry's trend of being notoriously unforgiving to an actress approaching 40.

The 39-year-old, who played Lydia Rodarte-Quayle on the hit US show, said that job offers had flooded in as she matured, with the industry only looking on the up for women.

She said: "I remember being really worried about (it becoming harder to get roles) in my early 30s and I've actually had more interesting roles and more frequent job offers than I did in my 20s.

"It is changing as well. I'm seeing more women in roles that used to be played by men - on a cop show, you know, it used to be like 80% men and now it feels like it's 70-30 or even 60-40 with women, so it feels like it's evening up a bit."

Proving her point, she has just finished filming a BBC four-parter called One Of Us, by the people behind The Missing.

"There's a great big double murder that I'm a detective trying to solve in the Highlands of Scotland, as you do," she laughed.